ON THE PROCESSING OF THE RECORDED DATA FOR THE SF6 CIRCUIT BREAKERS FROM THE TRANSFORMATION SUBSTATION

110/20/6kV CRAIOVA SOUTH

Maria Brojboiu Andrei Savescu Virginia Ivanov S.C. RELOC S.A.

University of Craiova 109 Decebal Blv., 200746, Craiova, Romania Facultaty of Electrical Engineering andreisavescu@hotmail.com

107 Decebal Blv., 200440, Craiova, Romania mbrojboiu@elth.ucv.ro, vivanov@elth.ucv.ro

KEYWORDS Circuit breakers, maintenance, SF6, ablation. ABSTRACT

The assurance of the reliability for the transformation substations is one of primary goals of the manufacturers and distributors of electricity. Therefore, a proper monitoring and maintenance program is required. The circuit breaker is a complex device, subject to the thermal and mechanical stresses during the normal or fault currents switching. The substations are frequently equipped with SF6 circuit breaker. The circuit breaker components subjected to the thermal stresses are the main contacts which suffer electrical erosion and the nozzle which is subjected to the ablation process. The ablation process appears because of the energy radiation which is transferred from the electric arc. As a result of the ablation, the nozzle geometry, the gas pressure and the electric withstand are changed. Based on the recorded data from the transformation substation 110/20/6kV Craiova South, the mass loss from nozzle, the admissible number of disconnections and the throat nozzle diameter are computed. The graphical representations highlight the impact of the interrupted current, of the arcing time and of the integral I2t over the mentioned ones. Consequently, if the switching process and the time arc value are controlled, the thermal wear can be limited and the equipments users may provide a maintenance program with minimal costs and an increasing of the lifetime. INTRODUCTION

The circuit breaker is one of the most important and complex equipment from the medium and high voltage electric substations having the switching functions of the electrical circuit in the normal or fault conditions. Depending on the thermal and dynamic stability of the electrical equipment from stations, the commutation must be carried out in a prescribed period of time. Consequently, the failure or decommissioning of circuit breaker has undesirable effects on the operation of the power station, thereby providing a program of monitoring, diagnosis and maintenance of the circuit

breaker it is absolutely necessary. The program of monitoring, diagnosis and maintenance has the purpose to increase the lifetime of the equipment and to reduce the operation and maintenance costs. From the maintenance costs of the electrical stations, a 40% are dedicated to the circuit breaker maintenance, meanwhile a 60% are dedicated to the general revisions ( Milthon S. et all, 2005). Therefore, the predictive maintenance systems based on the continuous monitoring of the circuit breaker lead to the significantly reducing of the costs. The predictive maintenance system has the advantage to be carried out during the operation of the equipment. A large number of references in the field are dedicated to the analysis of the functioning and monitoring of the circuit breakers (Milthon S. et all, 2003), (Richard, T., 2004), (Thanapong, S. 2006). The power stations from Craiova South are equipped with oil circuit breaker or, becoming frequently after upgrading, with SF6 circuit breakers. The use of this gas has reduced the frame sizes and increase performance s of switching. The medium voltage SF6 circuit breakers are designed of the self blast principle. This type of circuit breaker generates a gas flow by means of a piston and cylinder attached to the moving contact. When the circuit breaker is in close position the gas pressure from the puffer cylinder is equal to the pressure of filling gas. During the disconnecting operation, the SF6 gas is compressed in the cavity between puffer cylinder and the piston. The switching arc occurs between the stationary contact and the moving contact and it is develops inside of the blowing convergent- divergent nozzle from PTFE with a lower thermal conductivity. A successful current disconnecting depends on the interaction between the switching arc, the radiated energy from arc, the ablation of the nozzle material and the pressure of the gas flow. During the period of the current disconnection, in normal or fault regime, occurs the thermal wear of the circuit breaker components which are in contact with the switching arc (Richard, T., 2004), (Bang, H, 2012), (Bogatyreva, N, 2013),( Muratovic, M, 2013), (Weizong W.I., 2013). The components which are directly exposed to the radiative or conductive energy transferred from the switching arc are the electrical contacts and the blowing nozzle. As a result of the thermal wear, after one operation time or

Proceedings 28th European Conference on Modelling and Simulation ©ECMS Flaminio Squazzoni, Fabio Baronio, Claudia Archetti, Marco Castellani (Editors) ISBN: 978-0-9564944-8-1 / ISBN: 978-0-9564944-9-8 (CD)

after a cumulative number of disconnected currents, these components must be replaced. In the reference (Brojboiu, M. et all, 2013) the aspects of the contact electro erosion are presented and mass loss from the contacts because of the thermal erosion is computed. It is well known that there is an admissible limit of the thermal wear beyond which the circuit breaker operation cannot be assured and this fact requires the replacement of the used components. Having in view the thermal wear of the circuit breaker, the on line monitoring of the disconnections number, the arcing time and the disconnected current values allow the estimation of the circuit breaker condition and therefore a maintenance plan can be set out. Concerning the nozzle wear, during the operation, because of the electric arc presence, the ablation phenomenon of the nozzle material occurs. The electric energy arc is mainly absorbed by SF6 gas. An important part of this energy is absorbed by the contacts and nozzle. The energy absorption produces heating, melting and material vaporization, this being the main cause of the thermal wear. Following the ablation phenomenon occurs the increasing of the nozzle throat diameter and consequently the changing of the gas flow. In the same time, the mixing of the PTFE vapors (C2F4) with the SF6 gas appears. The influence of the vapors over the dielectric breakdown of the hot was analyzed in the reference (Weizong W.I., 2013) The PTFE vapors modify the properties of the quenching arc medium. Therefore, the quenching of the electric arc depends on the ablation intensity of the nozzle material. Consequently, the nozzle ablation has a significant influence over the composition of the gas or residual plasma between the main contacts, after the arc quenching. The withstand voltage of the gas - C2F4 vapors mixture is reduced in comparison with the one of the cold gas. The values of the critical electric field for various percentages of the PTFE vapors mixed with gas at a gas pressure of 0.40MPa were experimentally determined. Due to this fact, the re ignition of the electric arc can occurs and therefore a disconnection failure can happens. At the same time, the controlled ablation of the nozzle can produce the gas overpressure associated with the movement of a mechanical piston (Cae-Yoon B., et all, 2006). The severity of the thermal wear or the nozzle ablation depend on the amplitude of the disconnected current value, the electric arcing time, the integral I2t, the constructive solution of the circuit breaker and the recovery voltage amplitude arc (Richard, T., 2004). There are a large number of parameters that should be monitored to evaluate the circuit breaker condition during the operation time. Concerning the main contacts of the circuit breaker, the monitoring assumes the evaluation of the thermal erosion, electric arc duration, cumulative disconnected currents and the contact resistance. The monitoring of the nozzle implies the supervision of the inner diameter of the nozzle throat, the increasing pressure in the puffer chamber and the critical electric field value.

THE PROCESSING OF THE RECORDED DATA

The medium voltage SF6 circuit breaker, whose data were recorded (Savescu A. 2013), is Fluarc FG3 type, is installed in the transformer substation 110/20/6kV Craiova South - Ghercesti. The main rated values of this circuit breaker are: rated voltage 24kV, rated current 1250A, rated breaking capacity 25kA. The data were recorded using the protection system F650 General Electric and can be used for all equipment from transformation substations (oil or SF6 circuit breakers). Such protection system is able to store the last 20 events of the protected equipment. For circuit breakers, a recording contains the collected data in a range of time between one to two seconds. On this range of time, the currents, the voltages and the operating state of the circuit breaker (connected/disconnected/RAR) are graphically represented on every moment of time. The recorded data can be visualized by means the SIGRA4 program which is a part of the DIGSI software. The recording of variation in time of the interrupted current is shown in Figure 1. The current values are acquired from the secondary of a current transformer with transformation ratio 200/5. The arcing time is measured from the moment of time when the current falls below the value of 1A, at this value of arcing time, the post-arc currents occur between the main contacts of the circuit breaker. In the recording from figure 1, the time duration of the arc is of 24,3ms.

Figure 1: The Recorded Interrupted Currents vs. Time – L1,L2, L3 Phases of the SF6 Circuit Breaker

In the Figure 2 the recording of the variation in time of the voltages on the three L1,L2, L3 phases is shown. The recorded voltages are corresponding to the recorded interrupted currents. In this case, the transformation ratio of the voltage instrument transformer is of 1000/5. That means the voltage value in the moment of occurrence of the current interruption is about 22kV. The recorded interrupted currents values on three phases of the circuit breaker and the arcing time values are shown in Table 1.

Figure 2: The Recorded Voltages vs. Time – L1,L2, L3 Phases of the SF6 Circuit Breaker

Table 1: Recorded Values: Interrupted Currents, Arcing

Time / Computed Values: Mass Loss, Integral I2t

No. IL1 [A]

IL2 [A]

IL3 [A]

ta[ms]

m L1 [mg]

I2t⋅105

[A2s] 1 4200 4560 4360 27.3 500.0933 5.0943 2 4320 4400 4480 27.3 514.3816 5.3895 3 5000 4480 4320 27.4 597.5299 7.2471 4 1316 6.8 1332 22 178.5367 0.3548 5 2228 2216 14 32.8 318.7338 1.5506 6 896 904 6.8 25.7 146.8912 0.2118 7 832 852 25.6 49.2 100.4339 0.3459 8 592 7.2 4.8 27.3 70.4893 0.1012 9 1076 5.6 9.2 31.3 143.7840 0.3490

10 1088 7.2 5.6 30.3 146.8912 0.3551 11 728 7.2 5.2 26.3 83.5076 0.1455 12 2192 6.8 2200 33.3 318.3639 1.5330 13 2160 19.6 2164 27.5 259.0750 1.3573 14 2016 9.68 2020 27.3 240.0448 1.1737

The recorded data using the SIGRA 4 program must be combined and processed in order to make conclusions on the operation system and the circuit breaker state. That could help to increase the reliability of the electrical station. Because the circuit breakers must interrupt the fault currents throughout lifetime, their aging is due to the thermal and mechanical stresses of the blowing nozzle and the main contacts between the electric arc occurs. The thermal wear or cumulative electroerosion of the main contacts is used as criterion to evaluate the electrical endurance of the circuit breaker. Additionally, for the SF6 circuit breakers is very important to assess the cumulative wear of the blowing nozzle. Accordingly to (Thanapong, S. 2006), the total admissible electroerosion value of the main contacts and of the nozzle is depending on two parameters: the maximum breaking current Iscmax and the admissible number of disconnections of the fault currents Nadm. In the works (Brojboiu, M. et all, 2013), (Savescu, A.,2013) the computation of the mass loss from the main contacts depending on the recorded data in the oil circuit breaker from medium voltage substation Craiova is presented. The recorded data on the SF6 circuit breaker from the transformation substation 110/20/6kV Craiova

South has been processed in order to observe the dependencies between the mass loss from PTFE nozzle at the fault currents interruption and the electric arc energy, the integrals ∫idt and ∫i2dt. The computation of the electric arc energy was performed with formula:

aaa tIUW ⋅⋅= (1) where, the interrupted currents values and the arcing time values are presented in Table 1. The arc voltage drop Ua has been computed based on the described algorithm in (Hortopan, Gh., 1980). Knowing the maximum value of the recovery voltage as a function of maximum phase voltage, the amplitude factor ka, first phase factor kf, the oscillation frequency f, the following formula was deduced:

( )T/8a r maxU 0.707 u e− ⋅τ= ⋅ ⋅ (2)

where:

3/2kkUu fanmaxr ⋅⋅⋅= (3) is the maximum value of the recovery voltage, rated voltage Un=24kV, T=1/f, f is the oscillation frequency of the recovery voltage, τ is the time constant. In the work (Rong, M., 2005) it is estimated that 40% from electric arc power is used for the nozzle ablation, the ablation rate being around of kab=15…17mg/kJ. The mass loss mab as a function of the interrupted current value or the arc energy has been computed with formula:

aabab W4.0km ⋅⋅= (4) The variation of mass loss through ablation from PTFE nozzle depending on the interrupted currents values from three phases is shown in Figure 3.

0 1000 2000 3000 4000 5000 60000

0.1

0.2

0.3

0.4

0.5

0.6

0.7

Interrupted current [A]

Mas

slo

ss[g

]

Figure 3: The Mass Loss vs. Interrupted Current- L1,L2, L3 Phases

The processing of the experimental results was performed using a Matlab application. By applying a least squares approximation method to the recorded data, using a Matlab application, it was possible to plot a continuous curve (solid line) that approximates the variation of the mass loss from nozzle depending on the interrupted current. This Matlab application has been applied to all the recorded and computed data which were processed in this work. Concerning the computation of the integral I2t, one integration Matlab procedure has been applied. A sinusoidal variation of interrupted current was taken into account.

dt))tsin(I2(t2Iat

0

2∫ ω⋅⋅= (5)

In the Figure 4 the variation of the loss mass depending on the integral I2t is presented.

0 1 2 3 4 5 6 7 8

x 105

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

Integral I2t [A2s]

Mas

slo

ss[g

]

Figure 4: The Mass Loss vs. integral I2t From this figure, the increasing of the mass loss according to the increasing of the integral I2t values can be noticed, as it is expected. Using the values of nozzle mass loss and for a known geometry of the nozzle, the inner radius of the nozzle throat after ablation ra was calculated using the following formula:

(6)

where ri and h are the values of the internal radius and the height of the throat nozzle respectively, measured for the circuit breaker under measurements. For the PTFE as the material of the nozzle, the density value is taken as ρ=2200kg/m3.In the Figure 5 is graphically represented the variation of the ratio of the inner diameter after ablation to the initial inner diameter, depending on the amount of the mass lost from the throat nozzle. The large values of the ratio for reduced values of the mass loss can be observed. The experimental determinations carried out in reference (Thanapong, S.

2006) allow the establishing of the one empirical formula in order to estimate the limit mass loss depending on the admissible number of disconnections.

N94.20586.85Mlim ⋅+= (7) The admissible number of disconnections (the allowable number of disconnecting operations) is computed as a function of the ratio between the interrupted current and the maximum breaking current of the circuit k=I/Iscmax,Iscmax=25kA, using the following formula (Thanapong, S. 2006):

03245.1adm k4.4N −⋅= (8)

0 0.1 0.2 0.3 0.4 0.5 0.6 0.794

95

96

97

98

99

100

Mass loss [g]

Inne

rDia

met

ers

Rat

io[%

]

Figure 5: Inner Radius Ratio [%] vs. Mass Loss [g] In the Figure 6, the variation of the admissible number of disconnections depending on the ratio k, for the recorded current values of the L1 phase, is shown.

0.02 0.04 0.06 0.08 0.1 0.12 0.14 0.16 0.18 0.2 0.2220

40

60

80

100

120

140

160

180

200

220

Ratio k=I/Iscmax

Adm

issi

ble

disc

onne

ctio

nsnu

mbe

r

Figure 6: The Admissible Number of Disconnections vs. k=I/Iscmax

From the graphical representation, the decreasing of the admissible number of disconnections as the ratio k increases can be noticed.

( )h//mrr 1L2ia πρ⋅+=

CONCLUSIONS

The lifetime and the SF6 circuit breaker performances depend on the ablation intensity and the changes in the nozzle throat geometry, which are produced by the radiative and conductive energy of the electric arc. If the arcing time duration can be controlled and restricted in order to avoid the large values, the thermal wear of the contacts or the nozzle ablation can be limited. Consequently, the interval between maintenance activities can be extended. REFERENCES

Bang, H.; Lee ,Y. S.; Ahn, H. S.; Choi J. U.; Park, S. W. 2012. “A Study on Heat Transfer by Electric Arcs and Performance Prediction in Gas Circuit Breaker”. Recent Advances in Communications, Circuits and Technological Innovation. Paris.

Nadezhda, B.; Bartlova, M.; Aubrecht, V.; Holcman, V. 2013. “Mean Absorption Coefficients for SF6 + PTFE Arc Plasmas”, Electrorevue, ISSN 1213-1539, Vol.4, No.1, APRIL 2013.

Brojboiu, M.; Ivanov V.; Savescu, A. 2013. “Concerning the Monitoring of the Electric Contacts Electroerosion of the Circuit Breakers from Medium Voltage Stations of CEZ Craiova”, Annals of the University of Craiova, Serie Electrical Engineering, nr.37, ISSN 1842-4805, pp.124-127.

Himanshu, J,; Anjani, P.; Ghanashyam, P. 2013. “Optimization of High Voltage Arc Assist Interrupters”, International Journal of Scientific & Engineering Research, Vol. 4, Issue3, March-2013, ISSN 2229-5518.

Hortopan, Gh. 1980 “Aparate Electrice”, Editura Didactica si Pedagogica, Bucuresti.

Silva, M.; Jardini, J.; Magrini, L. 2005. “On-line condition monitoring system for in-service circuit breaker”, PEA. Department of Electrical Energy and Automation Engineering, Polytechnic School of the University of Sao Paulo, Brazil.

Silva, M.; Jardini, J.; Magrini, L. 2003. “An experience in circuit breaker on-line condition monitoring system design”, PEA - Department of Electrical Energy and Automation Engineering Polytechnic School of the University of Sao Paulo, Brazil.

Muratovic, M.; Kapetanovic, M.; Ahmethodzic, A.; Delic, S. 2013. “Calculation of nozzle ablation intensity and its influence on state of SF6 gas in thermal chamber”, Solid Dielectrics (ICSD), 2013 IEEE International Conference on, June 30 - July 4 2013, Bologna.

Richard, T. “Controlled Switching of High Voltage SF6 Circuit Breakers for Fault Interruption”, Thesis for the degree of licentiate of engineering.

Mingzhe, R.; Qian, Y.; Chunduo, F. 2005. “Simulation of the Process of Arc Energy-Effect in High Voltage Auto-Expansion SF6 Circuit Breaker”, Plasma Science and Technology, Vol. 7, Issue 6, pp. 3166-3169.

Savescu, A. 2013. “The maintenance of the 20kV circuit breaker- classical and modern constructive solutions”,

Thesis for the degree of licentiate of engineering, University of Craiova, Bilateral agreement for practical training with CEZ Craiova.

Suwanasri, T. 2006. “Investigation on No-loadMechanical Endurance and Electrical Degradation of a Circuit Breaker Model under Short Circuit Current Interruption”, Thesis.

Weizong, W. I. 2013. “Investigation of the dynamic characteristics and decaying behaviour of SF6 arcs in switching applications”, Thesis submitted to the University of Liverpool.

AUTHOR BIOGRAPHIES

MARIA BROJBOIU is currently working as Professor at the University of Craiova, Electrical Engineering Faculty, Department of Electrical Energetic and Aerospace Engineering. Before that, she worked as design

engineer at the Electroputere holding the Research and Development Center. She is Doctor in Science Technique – Electrical Engineering. She teaches the courses Electrical Equipment, Electrotechnologies and Industrial Systems Engineering. She published 5 books and 92 scientifical papers for different national and international conferences and symposiums.

VIRGINIA IVANOV was born in Vela, Dolj, Romania, 1963. She was graduated in Electrical Engineering at University of Craiova, Romania, in 1986 and Doctor in Electrical Engineering in 2004. From 1986 to 1998 she

worked as researcher with the Researching Institute for Motors, Transformers and Electric Equipment Craiova. In 1998 she joined the Faculty for Electrical Engineering, Department of Electrical Equipment and technologies. She is concerned with research activities in monitoring and modeling of electrical equipments.

ANDREI SAVESCU. He graduated bachelor studies in 2013 at the Faculty of Electrical Engineering at the University of Craiova and currently he is attending the Master's program "Energy quality and electromagnetic compatibility in electric systems". Since July 2013 he works as design engineer at RELOC

SA company from Craiova, which has as business line the maintenance and repair of locomotive as well as designing and construction activities concerning the new locomotive models.